(a) Field of the Invention
The present invention relates to a focusing method for a telephoto lens system and, more particularly, to a focusing method for a telephoto lens system of which the focal length is 180 mm and aperture ratio is large, i.e., F/1.8 or F/2.0.
(b) Description of the Prior Art
In recent years, there is increasing demand for large aperture ratio telephoto lens systems in various fields. A large aperture ratio telephoto lens system enables to take photographs at a high shutter speed. Therefore, when photographing a sport game, stage scene, etc., for example, it is possible to take photographs without causing object shaking and, consequently, the limitation in the photographing conditions is reduced. Besides, as the F-number with the full open aperture is small, it is possible to take a photograph in which the object is relieved by making the depth of field shallow and putting the things in front and rear of the object out of focus, such photography being characteristic of telephoto lens systems.
However, for a telephoto lens system with a large aperture ratio, the front lens group should be necessarily made large in diameter and should be composed of a large number of lens elements and, consequently, the front lens group becomes heavy. Therefore, in case of the conventional focusing method to advance the lens system as a whole, it is inconvenient for handling in various points, for example, the center of gravity shifts toward at the time of focusing, the helicoid becomes heavy becouse the amount of advancing movement should be made large as the focal length is long, and so forth.
Besides, the inner focusing method is also known as a focusing method for a telephoto lens system. In this method, it is difficult to keep the quality of image satisfactorily favourable up to a short distance. On the other hand, in case of the inner focusing method to move two lens components (for example, as disclosed in Japanese published examined patent application No. 39803/80), the power of one of the two lens components is weak and, consequently, the amount of movement of the other lens becomes large.